One of the disadvantages of conventional magnetic capture methods is that the magnetic separation is based on relatively weak magnetic field gradients, which in turn provide limited effectiveness, for example, in separating smaller magnetic particles from a fluid (e.g., a solution). For example, the existing magnetic capture methods and systems are generally limited to magnetic beads greater than 1 micrometer in diameter. In addition, the relatively weak magnetic field gradient limits the size of the tube and the volume of fluid that can be processed. For example, DynaMag2™ (Invitrogen, Grand Island, N.Y.) is designed to work with magnetic beads greater than 1 micrometer in diameter and is thus not effective with smaller magnetic beads, such as those in the 50 and 500 nanometer diameter range.
Many methods have been used to generate increased magnetic flux density gradients (Kang et al., Small 3, 1784-1791 (2007); and Xia et al., Biomed Microdevices 8, 299-308 (2006)), for example, using various microelectromechanical system (MEMS) technologies, but they require labor-intensive and time-consuming fabrication processes for structuring ferromagnetic materials at the nanometer to micrometer scale, such as photolithography, LIGA (Lithographie-Galvanoformung-Abformung/Lithography-Electroplating-Molding), and CMP (chemical mechanical polishing). The MACS magnetic column (Miltenyi®) (Miltenyi et al., Cytometry 11, 231-238 (1990)) can be used to trap smaller (e.g., 50 nm) magnetic particles. However, the MACS systems use steel wool and/or magnetizable wires packed into a column to accomplish magnetic gradient enhancement. However, the use of steel wool and/or magnetizable wires in a column makes the system harder to wash captured cells, prone to clogging, and/or prone to inducing clotting when used with blood. In addition, the throughput of the MACS systems is very limited (0.5 mL per a column). Due to the confined structures of the steel wool, it is difficult to apply the system to various experimental conditions or sample containers, such as tube or well plate configurations, fluidic devices (including microfluidic devices), etc. Accordingly, there is a need to develop novel and versatile methods, kits, devices, and systems for efficient and/or high throughput magnetic separation and/or capture of at least one or more target molecules from a fluid or solution.